The present invention is directed to gas turbine engines of the type having a low pressure compressor, a high pressure compressor downstream of the low pressure compressor, a combustor downstream of the high pressure compressor, a high pressure turbine downstream of the combustor, and a low pressure turbine downstream of the high pressure turbine. The standard configuration for engines of this type is a dual concentric shafting configuration wherein the high pressure turbine drives the high pressure compressor by means of a first shaft, and the low pressure turbine drives the low pressure compressor by means of a second shaft. Commercially available systems of this type are generally highly optimized so that the air and power flows through the compressors and turbines are closely matched. For example, the high pressure turbine generally supplies just the necessary power to drive the high pressure compressor. Similarly, the low pressure turbine supplies the necessary power to drive the low pressure compressor, the remaining power in the low pressure turbine being directed to useful work, i.e. to drive a load.
In one embodiment of such a gas turbine engine, the low pressure turbine is connected directly to the low pressure compressor and directly to the load. An example of such an engine is manufactured by General Electric of Evendale, Ohio, under the designation LM 6000. In another exemplary embodiment of a gas turbine engine of the type to which the present invention is directed, the low pressure turbine is connected only to the low pressure compressor, and a separate power turbine, located downstream of the low pressure turbine, is connected to the load. An example of such an engine is manufactured by General Electric Company of Evendale, Ohio, under the designation LM 5000.
One way to increase the output horsepower of such engines would be to simply burn more fuel. This procedure alone, however, would result in running the system at higher speeds and at higher temperatures than those for which it was designed, with the result being shortened life for the power plant. Taking an existing and optimized gas turbine of either of the types described above and adding to it an intercooler between the low and high pressure compressors would not, in and of itself, solve the problem. This intercooling would cause a reduction in the power requirements to the high pressure compressor, but the high pressure turbine would no longer operate at its designed pressure ratio and its performance would be degraded. In addition, the gas entering the low pressure turbine would be too hot and at too high a pressure, requiring flow function change and which, in turn, may result in reduced life and poor performance of the low pressure turbine.
The present invention is based upon the discovery that the output horsepower of engines of the type described can be significantly augmented by compression intercooling in combination with increasing the load on the high pressure turbine. In other words, if the power requirements of the high pressure compressor are reduced through intercooling by an amount approximating the power requirements of the low pressure compressor, then the high pressure turbine (the flow through which having been returned to approximately design temperature by additional fuel in the combustor) can be used to drive both compressors. The low pressure compressor is disconnected from the low pressure turbine and connected to the high pressure compressor. The low pressure compressor is generally designed to run at a lower speed than the high pressure compressor. When this is the case, the compressor can run at between about 1/5 and about 4/5 the speed of the high pressure compressor. In many instances the low pressure compressor runs at a speed of from about 1/3 to about 1/2 that of the high pressure compressor. Thus, when the low pressure compressor is designed to run slower than the high pressure compressor, a speed reduction gear box is required in the drive train between the high pressure compressor and the low pressure compressor. As a result of the above, the low pressure turbine is no longer required to drive the low pressure compressor and can be disconnected therefrom. Accordingly, all of the output of the low pressure turbine can be used to drive the attached load directly, or through the agency of a power turbine, if present. Furthermore, since the cooling air to the high pressure turbine will be cooler, the temperature of the flow from the combustor to the high pressure turbine can be increased to increase the efficiency of the engine and its output.
In this fashion, it is possible to take a commercially available multi-shaft gas turbine engine and intercool it in a straightforward and cost effective manner, and therefore achieve the increased power output and efficiency of an intercooled engine. Furthermore, output horsepower improvements of from about 20% to about 40% or more can be achieved without making major changes in the aerodynamic design of the engine or changes in the core hardware which would entail costly development and time-consuming testing. The practice of the present invention would not only enable the building of new engines of improved output horsepower utilizing a preponderance of production parts, but also the conversion of engines in the field to achieve greater output horsepower.